The Castle Hill High School April Fool's Day prankster has struck again.

This morning students and their parents woke up to a hoax text message claiming there had been a fire at the school and it had closed.

At 11.37am, parents and students received a second text message from the prankster.

It said: "Hey there - As you may already know, the previous messages were sent as part if an April Fools Prank and school did go on as usual. I understand how this may have inconvenienced some people, but you really should have looked at your calendar before believing."

A Department of Education spokesman said the school's text messaging system has now been shut down.

"The school is not going to use the system until it's okay," he said.

"Communication with parents will be through emails and the school's website."

One student described the first text message as a "good prank".

"Whoever has done this is quite the genius and I take my hat off to that person if I had a hat on.

"I think parents and the media shouldn't take this so seriously considering it was rectified before school started.

"Good on them because it shows a flaw in the security system."

Earlier this morning, a department spokesman told Fairfax Media the school is open today and year 12 mid-year exams are on.

"Students might be concerned about their exams,'' he said.

''It just shows that the practical joke not only affects the parents but also the students.

''Because the IT system has been interfered with, the matter has been referred to the police.''

Hills crime manager Detective Inspector Gary Bailey said he had never seen anything like this in The Hills before.

He said officers were reviewing the call charges on the school’s bill.

‘‘Then on their system we do some inquiries through their IT section,’’ Detective Inspector Bailey said.

He said about 1100 SMS’s are being sent out each time.

‘‘There was a bodgy message sent out at 2.31am and then the legitimate one to correct, saying they are still required to attend [school], went out at 2.45am,’’ he said.

‘‘Later on at 3.07am the message was not authorised and there was a message to say it was an April Fool.

‘‘They [the SMS’s] were routed through the same number.

‘‘Castle Hill has done everything as quickly as they can to inform everyone it [the original SMS] wasn’t right.’’

Detective Inspector Bailey said the investigation has used a lot of police resources.

‘‘It’s interfered with the public,’’ he said.‘‘It could also be an offence under the Commonwealth Crimes Act. ''There may be something the school needs to look at. It may be you can access this [system] at home.’’

Students responded this morning on Facebook to the prank.

One student said on Facebook: ''Not sure if school burned down or best April fools joke ahahha.''

Another student responded: ''Guys, the text that went out to everyone is an April Fools joke. Word from Kerro, and a mention on the news, says so. Sorry to be the spoilsport, but you're all still expected at school.''

The department spokesman said an investigation will have to take place to ensure that confidential information about students has not been accessed.